Hitachi ULTRASTAR 15K450 HUS154530VLF400 - Manual (Page 235)

Hitachi Ultrastar 15K450 (FC-AL) Hard Disk Drive Specification
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19.20.4 Read Data from Echo Buffer (Mode 01010b)
In this mode the drive transfers data from the echo buffer. The echo buffer will transfer the same data as when the WRITE
BUFFER command was issued with the mode field set to echo buffer.
WRITE BUFFER command with the mode field set to echo buffer should be sent prior to the READ BUFFER command; oth-
erwise the READ BUFFER command will be terminated with Check Condition status and Illegal Request.
In this mode Read Buffer transfers the specified amount of data or the amount previously written with a Write Buffer using
mode 1010b from the echo buffer, whichever is less.
Issuing a Read Buffer mode 1010b before a Write Buffer mode 1010b will cause indeterminate data to be returned.
The most significant two bytes of the Allocation Length are ignored. The specified amount of data transferred should not be
larger than the echo buffer capacity. The echo buffer capacity may be determined by using Read Buffer mode 1011b. Any
additional data transferred over and above the echo buffer capacity is regarded as indeterminate.
The Buffer ID and Buffer Offset fields are ignored in this mode.
Note: The echo buffer is a separate buffer from the data buffer used with other read buffer modes. It is intended to be used
for domain validation purposes.